A straight-line DCA for a gas reservoir

Abstract

In decline curve analysis (DCA), the decline exponent (b) characterizes the variation of decline rate (D) with production, and significantly affects the long-term shape of the production profile. In most currently-available DCA methods, the decline exponent is assumed to be constant during the whole production period. However, production decline of a gas reservoir does not follow exponential (b = 0), hyperbolic (0 < b < 1), or harmonic (b = 1) decline profiles, and instead the value of b varies with production time. Application of current DCA methods to predict future performance yields erroneous results, especially for high-pressure gas reservoirs.The objective of this paper is to propose a new method in which the effect of depletion-related changes in the value of b is taken into account. The new methodology can be applied to analyze the production data from a gas well during the entire boundary dominated production period, allowing Gas Initially-In-Place (G) to be estimated at any stage of that period. The proposed methodology is validated using both synthetic and field examples.